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CALCULATION OF CROP WATER REQUIREMENTS USING

CROPWAT

BY: MARIE THERESE ABI SAAB - NOVEMBER 2015

HOW TO IRRIGATE ?

HOW MUCH TO IRRIGATE ?

WHEN TO IRRIGATE ?

THE FUNDAMENTAL QUESTIONS OF IRRIGATION

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CROPWAT IS A DECISION SUPPORT SYSTEM DEVELOPED BY FAO, HAVING AS MAIN FUNCTIONS: to calculate: reference evapotranspiration, crop water

requirements, crop irrigation requirements; to develop: irrigation schedules under various management

conditions, scheme water supply;

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http://www.fao.org/nr/water/infores_databases.html

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PROGRAM STRUCTURE

five data input modules three calculation modules

THE INPUT MODULES

Climate/Eto

Rain

Crop

Soil

Crop pattern

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INPUT the water balance method is used for calculation of irrigation schedules in cropwat, which means that the incoming and outgoing water flows from the soil profile are monitored.

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CR

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SETTINGS MENU

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SETTINGS MENU

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INSERT ALL REQUIRED INPUTS

1. Climate (Et0, rain)

2. Crop

3. Soil

4. Crop Pattern

Climate

ETo (mm) and Pe (mm)

1 mm = 1 L/m2 = 1000 L/du = 1 m3/du

Climate data: linked/not linked to a specific year

Monthly data (serve to get an idea about a potential schedule to be

recommended for farmers in some area

Daily data: real time irrigation monitoring

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Any proposed climate database???

Import climate files to CROPWAT

EFFECTIVE RAINFALL

To account for the losses due to runoff or percolation, a

choice can be made of one of the four methods given in CROPWAT 8.0 (Fixed

percentage, Dependable rain, Empirical formula, USDA Soil Conservation

Service).

In general, the efficiency of rainfall will decrease with increasing rainfall. For

most rainfall values below 100 mm/month, the efficiency will be approximately

80%. Unless more detailed information is available for local conditions, it is

suggested to select the Option Fixed percentage and give 80% as requested

value.

In the water balance calculations included in the irrigation scheduling part of

CROPWAT, a possibility exists to evaluate actual Efficiency values for different

crops and soil conditions.

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EFFECTIVE RAINFALL

CROP MODULE

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SOIL DATA

The soil parameters necessary for irrigation scheduling for CROPWAT program are:

1. Total available soil moisture content (SMta), defined as the difference in soil

moisture content between field capacity (FC) and wilting point (PWP). This is

the total amount of water available to the crop and depends on texture, structure

and organic matter content;

2. Initial soil moisture depletion indicates the dryness of the soil at the start of

irrigation. This is expressed as a depletion percentage from FC;

3. Maximum rooting depth will in most cases be determined by the genetic

characteristics of the plant. In some cases the root depth can be restricted by

limiting layers ;

4. Maximum rain infiltration rate allows for an estimate of the surface runoff for

the effective rain calculation. This is a function of rain intensity, soil type and

slope class.

THE SOIL MODULE IS ESSENTIALLY DATA INPUT, REQUIRING THE FOLLOWING GENERAL SOIL DATA:

Total Available Water ( TAW)

Maximum infiltration rate

Maximum rooting depth

Initial soil moisture depletion

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DEMONSTRATION ON THE SOFTWARE !